1. Field of the Invention
The present invention relates to a sample tube used to measure nuclear magnetic resonance (NMR) spectra of solid samples. The invention also relates to a method of solid-state NMR measurements using this sample tube.
2. Description of Related Art
An NMR (nuclear magnetic resonance) spectrometer is an analytical instrument for detecting signals from atomic nuclei having spin magnetic moments by applying a static magnetic field to the atomic nuclei, producing a Larmor precession on the spin magnetic moments, and applying an RF radiation having the same frequency as the precession to induce a resonance.
Samples to be investigated by NMR include two types: solution samples and solid samples. Many solution samples give quite sharp NMR spectra and, therefore, it is widespread to perform molecular structural analysis of chemical substances by utilizing the excellent performance of the obtained high-resolution NMR spectra.
On the other hand, in an NMR spectrum of a sample in solid phase, interactions (such as dipolar interactions) which would be nullified by rotational Brownian motion in a solution manifest themselves directly and so the spectral line width broadens extremely, thus, obscuring chemical shift terms. Therefore, in such an NMR spectrum, it is impossible to isolate the signal peaks arising from various portions of a molecule under investigation. As a result, it has been thought that solid-state NMR spectroscopy is unsuited for molecular structure analysis.
A method which overcomes this undesired phenomenon and gives rise to sharp solid-state NMR spectra was discovered by E. R. Andrew in 1958. In particular, anisotropic interactions are removed and chemical shift terms can be extracted by tilting the sample tube at an angle of 54.7° to the direction of the static magnetic field B0 and spinning the tube at high speed. This method is known as MAS (magic angle spinning).
A solid-state NMR instrument includes a mechanism for adjusting the angle of the axis of rotation of a sample tube. The mechanism is shown in the block diagram of FIG. 1. The instrument has a probe 1. The sample tube 2 holds a sample therein. The tube 2 is inserted in a spinner stator 3 and spun at high speed using a gaseous medium such as compressed air or nitrogen gas.
A movable mechanism 4 such as a toothed wheel is used to vary the angle of the spinner stator 3. A shaft 5 or the like is connected to the movable mechanism 4 to permit the movable mechanism 4 to be controlled from outside. A knob 6 that is connected with the shaft 5 is accessed and manipulated by a user when the magic angle is actually adjusted.
Chemical shift anisotropy can be eliminated and the NMR spectral line width can be sharpened by spinning the sample tube at the magic angle of 54.7° to the static magnetic field B0. Therefore, adjustment of the magic angle is an important technique.
Generally, in order to investigate solid samples by MAS NMR, the sample must be spun at a high speed from a few kHz to tens of kHz within a static magnetic field. Accordingly, to obtain such rotational speeds, gas bearing techniques have been heretofore used, and various methods have been proposed.
FIG. 2 shows one example of the positional relationship between a sample tube A1 for holding a solid sample to be spun at high speed, radial gas bearings A2 supporting the tube A1, and a thrust gas bearing A3 acting to determine the position of the tube A1 in the thrust direction. The sample tube A1 is floated by feeding air into the radial gas bearings A2 and thrust gas bearing A3 and is kept out of contact with the surroundings. The sample tube A1 is spun at high speed by ejecting an air jet at a turbine A5 from a nozzle A4.
FIGS. 3 and 4 schematically illustrate a bending resonance of a sample tube B1 when it is spun at high speed. The natural frequency of the bending resonance of the tube B1 becomes lower as the diameter of the sample tube B1 decreases (e.g., where the diameter is less than 1 mm).
Therefore, if the rotational speed of the sample tube B1 is increased, the natural frequency of bending mode of the sample tube B1 is approached. The sample tube B1 comes into a bending resonance at some rotational frequency. As a result, as shown in FIG. 4, the sample tube B1 in a bending resonance comes into contact with radial gas bearings B2, thus creating the problem that the rotational speed of the sample tube B1 is restricted.
Generally, high-speed rotation of a sample tube is effectively achieved by lowering the peripheral speed of the sample tube when it is rotating so as to lessen the effects of the viscosity of the gas. One means conventionally adopted for this purpose is to reduce the outside diameter of the sample tube.
However, sufficient consideration has not been given to the material and length of the sample tube and, therefore, the target rotational speed of the sample tube approaches or exceeds the natural frequency of bending mode, resulting in instability of the rotation of the tube or its fracture. Consequently, high-speed rotation corresponding to the diameter of the sample tube has not been accomplished.